NAME

SYNOPSIS

Table of Contents

Encode consists of a collection of modules whose details are too extensive
to fit in one document. This one itself explains the top-level APIs
and general topics at a glance. For other topics and more details,
see the documentation for these modules:

DESCRIPTION

The Encode
module provides the interface between Perl strings
and the rest of the system. Perl strings are sequences of
characters.

The repertoire of characters that Perl can represent is a superset of those
defined by the Unicode Consortium. On most platforms the ordinal
values of a character as returned by ord(S) is the Unicode
codepoint for that character. The exceptions are platforms where
the legacy encoding is some variant of EBCDIC rather than a superset
of ASCII; see perlebcdic.

During recent history, data is moved around a computer in 8-bit chunks,
often called "bytes" but also known as "octets" in standards documents.
Perl is widely used to manipulate data of many types: not only strings of
characters representing human or computer languages, but also "binary"
data, being the machine's representation of numbers, pixels in an image, or
just about anything.

When Perl is processing "binary data", the programmer wants Perl to
process "sequences of bytes". This is not a problem for Perl: because a
byte has 256 possible values, it easily fits in Perl's much larger
"logical character".

TERMINOLOGY

character

A character in the range 0 .. 2**32-1 (or more);
what Perl's strings are made of.

byte

A character in the range 0..255;
a special case of a Perl character.

octet

8 bits of data, with ordinal values 0..255;
term for bytes passed to or from a non-Perl context, such as a disk file,
standard I/O stream, database, command-line argument, environment variable,
socket etc.

THE PERL ENCODING API

Basic methods

encode

Encodes the scalar value STRING from Perl's internal form into
ENCODING and returns a sequence of octets. ENCODING can be either a
canonical name or an alias. For encoding names and aliases, see
Defining Aliases. For CHECK, see Handling Malformed Data.

For example, to convert a string from Perl's internal format into
ISO-8859-1, also known as Latin1:

$octets = encode("iso-8859-1",$string);

CAVEAT: When you run $octets = encode("utf8",$string)
, then
$octets might not be equal to $string. Though both contain the
same data, the UTF8 flag for $octets is always off. When you
encode anything, the UTF8 flag on the result is always off, even when it
contains a completely valid utf8 string. See The UTF8 flag below.

decode

This function returns the string that results from decoding the scalar
value OCTETS, assumed to be a sequence of octets in ENCODING, into
Perl's internal form. As with encode(),
ENCODING can be either a canonical name or an alias. For encoding names
and aliases, see Defining Aliases; for CHECK, see Handling Malformed Data.

For example, to convert ISO-8859-1 data into a string in Perl's
internal format:

$string = decode("iso-8859-1",$octets);

CAVEAT: When you run $string = decode("utf8",$octets)
, then $string
might not be equal to $octets. Though both contain the same data, the
UTF8 flag for $string is on. See The UTF8 flag
below.

from_to

Converts in-place data between two encodings. The data in $octets
must be encoded as octets and not as characters in Perl's internal
format. For example, to convert ISO-8859-1 data into Microsoft's CP1250
encoding:

from_to($octets,"iso-8859-1","cp1250");

and to convert it back:

from_to($octets,"cp1250","iso-8859-1");

Because the conversion happens in place, the data to be
converted cannot be a string constant: it must be a scalar variable.

from_to()
returns the length of the converted string in octets on success,
and undef on error.

CAVEAT: The following operations may look the same, but are not:

from_to($data,"iso-8859-1","utf8");#1

$data = decode("iso-8859-1",$data);#2

Both #1 and #2 make $data consist of a completely valid UTF-8 string,
but only #2 turns the UTF8 flag on. #1 is equivalent to:

Yes, it does not respect the $check during decoding. It is
deliberately done that way. If you need minute control, use decode
followed by encode
as follows:

$octets = encode($to,decode($from,$octets,$check_from),$check_to);

encode_utf8

$octets = encode_utf8($string);

Equivalent to $octets = encode("utf8",$string)
. The characters in
$string are encoded in Perl's internal format, and the result is returned
as a sequence of octets. Because all possible characters in Perl have a
(loose, not strict) UTF-8 representation, this function cannot fail.

decode_utf8

Equivalent to $string = decode("utf8",$octets [,CHECK])
.
The sequence of octets represented by $octets is decoded
from UTF-8 into a sequence of logical characters.
Because not all sequences of octets are valid UTF-8,
it is quite possible for this function to fail.
For CHECK, see Handling Malformed Data.

Finding IANA Character Set Registry names

The canonical name of a given encoding does not necessarily agree with
IANA Character Set Registry, commonly seen as Content-Type:
text/plain; charset=WHATEVER. For most cases, the canonical name
works, but sometimes it does not, most notably with "utf-8-strict".

As of Encode
version 2.21, a new method mime_name()
is therefore added.

Fortunately, all encodings that come with Encode
core are PerlIO
-savvy
except for hz
and ISO-2022-kr
. For the gory details, see
Encode::Encoding and Encode::PerlIO.

Handling Malformed Data

The optional CHECK argument tells Encode
what to do when
encountering malformed data. Without CHECK, Encode::FB_DEFAULT
(== 0) is assumed.

As of version 2.12, Encode
supports coderef values for CHECK;
see below.

NOTE: Not all encodings support this feature.
Some encodings ignore the CHECK argument. For example,
Encode::Unicode ignores CHECK and it always croaks on error.

List of CHECK values

FB_DEFAULT

I<CHECK> = Encode::FB_DEFAULT( == 0)

If CHECK is 0, encoding and decoding replace any malformed character
with a substitution character. When you encode, SUBCHAR is used.
When you decode, the Unicode REPLACEMENT CHARACTER, code point U+FFFD, is
used. If the data is supposed to be UTF-8, an optional lexical warning of
warning category "utf8"
is given.

FB_CROAK

I<CHECK> = Encode::FB_CROAK( == 1)

If CHECK is 1, methods immediately die with an error
message. Therefore, when CHECK is 1, you should trap
exceptions with eval{}, unless you really want to let it die.

FB_QUIET

I<CHECK> = Encode::FB_QUIET

If CHECK is set to Encode::FB_QUIET
, encoding and decoding immediately
return the portion of the data that has been processed so far when an
error occurs. The data argument is overwritten with everything
after that point; that is, the unprocessed portion of the data. This is
handy when you have to call decode
repeatedly in the case where your
source data may contain partial multi-byte character sequences,
(that is, you are reading with a fixed-width buffer). Here's some sample
code to do exactly that:

FB_WARN

This is the same as FB_QUIET
above, except that instead of being silent
on errors, it issues a warning. This is handy for when you are debugging.

FB_PERLQQ FB_HTMLCREF FB_XMLCREF

perlqq mode (CHECK = Encode::FB_PERLQQ)

HTML charref mode (CHECK = Encode::FB_HTMLCREF)

XML charref mode (CHECK = Encode::FB_XMLCREF)

For encodings that are implemented by the Encode::XS
module, CHECK==Encode::FB_PERLQQ
puts encode
and decode
into perlqq
fallback mode.

When you decode, \xHH is inserted for a malformed character, where
HH is the hex representation of the octet that could not be decoded to
utf8. When you encode, \x{HHHH} will be inserted, where HHHH is
the Unicode code point (in any number of hex digits) of the character that
cannot be found in the character repertoire of the encoding.

The HTML/XML character reference modes are about the same. In place of
\x{HHHH}, HTML uses &#NNN; where NNN is a decimal number, and
XML uses &#xHHHH; where HHHH is the hexadecimal number.

In Encode
2.10 or later, LEAVE_SRC
is also implied.

The bitmask

These modes are all actually set via a bitmask. Here is how the FB_XXX
constants are laid out. You can import the FB_XXX constants via
useEncodeqw(:fallbacks)
, and you can import the generic bitmask
constants via useEncodeqw(:fallback_all)
.

FB_DEFAULTFB_CROAKFB_QUIETFB_WARNFB_PERLQQ

DIE_ON_ERR0x0001X

WARN_ON_ERR0x0002X

RETURN_ON_ERR0x0004XX

LEAVE_SRC0x0008X

PERLQQ0x0100X

HTMLCREF0x0200

XMLCREF0x0400

LEAVE_SRC

Encode::LEAVE_SRC

If the Encode::LEAVE_SRC
bit is not set but CHECK is set, then the
source string to encode() or decode() will be overwritten in place.
If you're not interested in this, then bitwise-OR it with the bitmask.

coderef for CHECK

As of Encode
2.12, CHECK can also be a code reference which takes the
ordinal value of the unmapped character as an argument and returns
octets that represent the fallback character. For instance:

Even the fallback for decode
must return octets, which are
then decoded with the character encoding that decode
accepts. So for
example if you wish to decode octets as UTF-8, and use ISO-8859-15 as
a fallback for bytes that are not valid UTF-8, you could write

Defining Encodings

CANONICAL_NAME will be associated with $object. The object
should provide the interface described in Encode::Encoding.
If more than two arguments are provided, additional
arguments are considered aliases for $object.

The UTF8 flag

Before the introduction of Unicode support in Perl, The eq operator
just compared the strings represented by two scalars. Beginning with
Perl 5.8, eq compares two strings with simultaneous consideration of
the UTF8 flag. To explain why we made it so, I quote from page 402 of
Programming Perl, 3rd ed.

Goal #1:

Old byte-oriented programs should not spontaneously break on the old
byte-oriented data they used to work on.

Goal #2:

Old byte-oriented programs should magically start working on the new
character-oriented data when appropriate.

Goal #3:

Programs should run just as fast in the new character-oriented mode
as in the old byte-oriented mode.

Goal #4:

Perl should remain one language, rather than forking into a
byte-oriented Perl and a character-oriented Perl.

When Programming Perl, 3rd ed. was written, not even Perl 5.6.0 had been
born yet, many features documented in the book remained unimplemented for a
long time. Perl 5.8 corrected much of this, and the introduction of the
UTF8 flag is one of them. You can think of there being two fundamentally
different kinds of strings and string-operations in Perl: one a
byte-oriented mode for when the internal UTF8 flag is off, and the other a
character-oriented mode for when the internal UTF8 flag is on.

Here is how Encode
handles the UTF8 flag.

When you encode, the resulting UTF8 flag is always off.

When you decode, the resulting UTF8 flag is on--unless you can
unambiguously represent data. Here is what we mean by "unambiguously".
After $utf8 = decode("foo",$octet)
,

As you see, there is one exception: in ASCII. That way you can assume
Goal #1. And with Encode
, Goal #2 is assumed but you still have to be
careful in the cases mentioned in the CAVEAT paragraphs above.

This UTF8 flag is not visible in Perl scripts, exactly for the same reason
you cannot (or rather, you don't have to) see whether a scalar contains
a string, an integer, or a floating-point number. But you can still peek
and poke these if you will. See the next section.

Messing with Perl's Internals

The following API uses parts of Perl's internals in the current
implementation. As such, they are efficient but may change in a future
release.

_utf8_on

_utf8_on(STRING)

[INTERNAL] Turns the STRING's internal UTF8 flag on. The STRING
is not checked for containing only well-formed UTF-8. Do not use this
unless you know with absolute certainty that the STRING holds only
well-formed UTF-8. Returns the previous state of the UTF8 flag (so please
don't treat the return value as indicating success or failure), or undef
if STRING is not a string.

NOTE: For security reasons, this function does not work on tainted values.

_utf8_off

_utf8_off(STRING)

[INTERNAL] Turns the STRING's internal UTF8 flag off. Do not use
frivolously. Returns the previous state of the UTF8 flag, or undef if
STRING is not a string. Do not treat the return value as indicative of
success or failure, because that isn't what it means: it is only the
previous setting.

NOTE: For security reasons, this function does not work on tainted values.

UTF-8 vs. utf8 vs. UTF8

That has historically been Perl's notion of UTF-8, as that is how UTF-8 was
first conceived by Ken Thompson when he invented it. However, thanks to
later revisions to the applicable standards, official UTF-8 is now rather
stricter than that. For example, its range is much narrower (0 .. 0x10_FFFF
to cover only 21 bits instead of 32 or 64 bits) and some sequences
are not allowed, like those used in surrogate pairs, the 31 non-character
code points 0xFDD0 .. 0xFDEF, the last two code points in any plane
(0xXX_FFFE and 0xXX_FFFF), all non-shortest encodings, etc.

The former default in which Perl would always use a loose interpretation of
UTF-8 has now been overruled:

Got that? As of Perl 5.8.7, "UTF-8" means UTF-8 in its current
sense, which is conservative and strict and security-conscious, whereas
"utf8" means UTF-8 in its former sense, which was liberal and loose and
lax. Encode
version 2.10 or later thus groks this subtle but critically
important distinction between "UTF-8"
and "utf8"
.

encode("utf8","\x{FFFF_FFFF}",1);# okay

encode("UTF-8","\x{FFFF_FFFF}",1);# croaks

In the Encode
module, "UTF-8"
is actually a canonical name for
"utf-8-strict"
. That hyphen between the "UTF"
and the "8"
is
critical; without it, Encode
goes "liberal" and (perhaps overly-)permissive:

find_encoding("UTF-8")->name# is 'utf-8-strict'

find_encoding("utf-8")->name# ditto. names are case insensitive

find_encoding("utf_8")->name# ditto. "_" are treated as "-"

find_encoding("UTF8")->name# is 'utf8'.

Perl's internal UTF8 flag is called "UTF8", without a hyphen. It indicates
whether a string is internally encoded as "utf8", also without a hyphen.

MAINTAINER

This project was originated by the late Nick Ing-Simmons and later
maintained by Dan Kogai <dankogai@cpan.org>. See AUTHORS
for a full list of people involved. For any questions, send mail to
<perl-unicode@perl.org> so that we can all share.

While Dan Kogai retains the copyright as a maintainer, credit
should go to all those involved. See AUTHORS for a list of those
who submitted code to the project.

COPYRIGHT

Copyright 2002-2014 Dan Kogai <dankogai@cpan.org>.

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.